Recent developments in our laboratories have shown that specifically designed molecular probes can afford a great deal of detailed information regarding the mechanism of the liver microsomal cytochrome P-450-mediated oxidation of organic compounds. Deuterium labeling techniques and monitoring of molecular rearrangements, which have been developed over several years with a model system, will be applied to the enzyme to determine the role and nature of carbon-free radicals that appear to be generated in the course of the catalytic cycle. The fate of these radicals may be intimately involved with the known hepatotoxicity of a large number of compounds.